Sorry because I cannot comment...
I had the same problem. In my case the problem was related to trying to restore in an old sql server folder (that existed on the server). This is due to old sql server backup (i.e. SQL Server 2012 Backup) restored in a new sql server (SQL Server 2014). The real issue is not too different from @marc_s answer. Anyway, I changed only the target folder to the new SQL Server DATA folder.